The present invention relates to fixtures including aerial markers and spacers for elevated cables, such as power transmission lines, for preventing collisions between nearby aircraft and the lines, and for facilitating maintenance of the lines.
Power lines are typically suspended from spaced towers, the towers being widely spaced apart where the lines extend across canyons, rivers, and the like. In such cases the lines have substantial elevation over the ground between the towers and thus present hazardous conditions for aircraft. Consequently, various line markers have been developed to reduce such hazards. For example, U.S. Pat. No. 4,885,835 to Osgood discloses a power-line marker having a pair of flanged hemispheric shells that are assembled on opposite sides of the line and anchored thereto by wire strands that are spirally twisted about the line.
U.S. Pat. No. 3,362,377 to Hill et al. discloses a span marker having nested spherical shell segments that are movable between an open position for receiving the line and a closed position clamping the line. An installation tool that is manipulated from a helicopter is used for lowering the marker onto the line and removing a locking pin that temporarily holds the shell segments open against a biasing spring. Removal of the marker, if desired, is effected by hooking onto and retracting another locking pin that anchors the biasing spring, and that is tethered to one of the shell segments.
These and other line markers of the prior art exhibit a number of disadvantages, such as one or more of the following:
1. They are difficult to install in that they require assembly in place on the line by means of fasteners and the like, and in case of installation by helicopter, they require multiple crew members; PA0 2. They are difficult or impractical to remove from the line or relocated on the line because of required disassembly, because clamping mechanisms thereof are disabled by being released, or because the clamping mechanisms become vulcanized onto the line; PA0 3. They are limited in application to a narrow range of line sizes; PA0 4. They are ineffective in that they are subject to being moved out of place on the line in the presence of wind loading, particularly in cases when the markers are not concentrically located on the line; and PA0 5. They are unreliable in that they have clamp mechanisms that are subject to high levels of frictional resistance to clamping in relation to available clamping forces.
Other fixtures for transmission lines include suspended spacers for maintaining grouped conductors of transmission lines in a predetermined closely spaced relation. Spacers of the prior art are subject to many of the above disadvantages of the markers, particularly in that installation is complicated by the need to engage a plurality of conductors.
Thus there is a need for line fixtures and a system for installing and maintaining same that is effective, safe and inexpensive to produce and use.